1. Field of the Invention
The invention relates to a pressure vessel arrangement which comprises an external pressure vessel and at least one insert basket arranged therein for receiving a substance to be treated. Said substance is exposable to a pressure fluid which is feedable to the pressure vessel and the insert basket.
2. Background Art
With respect to the background of the invention, it shall be noted that the invention is primarily relevant when treating solids with liquids and/or gases. The substance to be treated is filled into a pressure vessel which needs to withstand the pressures and temperatures required for treatment.
‘Treatment’ in this context refers to mechanical or thermal separation or cleaning processes. Examples thereof include filtering, elutriation, extraction or adsorption.
The substances to be treated may be directly filled into the vessels and removed therefrom in a suitable manner once the process is finished. The processes of filling and emptying are simple if the solid is flowable prior to and after treatment.
A plurality of fasteners such as so-called quick-release fasteners have been developed for faster filling and emptying. In the field of high-pressure application engineering, the so-called quick-action clamp has become common standard for vessels with smaller diameters. Clamps for vessels with larger diameters would become too heavy; therefore, other designs such as the ‘Brettschneider’ fastener have been developed.
It is common practice as well, in particular when using smaller vessels, to use insert baskets. The advantages thereof need to be weighed against the respective production costs:
Filling and emptying may be performed outside the pressure vessel. Advantages are particularly evident when using explosive dusts and vapors. The same applies in the processing of solids where careful attention has to be paid to the hygienic conditions. Due to these operating limitations, it may be mandatory to use baskets.
A purely economic advantage is that a plurality of insert baskets can be prepared at suitable times so that they only need to be exchanged during the mostly understaffed night shifts, for example.
The solid provided in the insert baskets can be flown through (axially) from the bottom to the top or vice versa in the longitudinal direction of the vessel but also from the outside to the inside and vice versa (radially). The second mode of operation is particularly advantageous if slow flow rates are desired, for instance during adsorption.
In the following, the term ‘treatment’ refers more specifically to the extraction of solids using liquid or supercritical gases. Carbon dioxide is preferably used. The invention is however by no means limited to this application.
The typical extraction procedure is described using the example of hops.
Prior to extraction, hops are cleaned to remove heavy particles before being coarsely ground and pressed into pellets. Once cooled down, pellets are weighed and filled into flexible containers. These containers are transported to the respective extraction installation, the pellets are filled into the pressure vessel directly but may also be filled into insert baskets.
Extraction takes place in the same way in all installations. The solvent flows through the bed of hops pellets at high pressure so as to be loaded with the significant components of the hops. In the following separation step, the previously dissolved components are separated from the solvent and discharged form the pressure vessel. Extraction of the pellets occurs in batches as the pressure in the respective vessel needs to be reduced to atmospheric pressure when removing and filling in the pellets.
When the pressure in the pressure vessel is reduced to atmospheric pressure, this leads to serious problems as far as the vessel fasteners and insert baskets are concerned. The pressure vessels (extraction and separation vessels) are equipped with quick-release fasteners. A two-component clamp is used to secure the vessel lid to the vessel. A seal prevents the escape of gas. In order to open the lid, the two clamp halves need to be moved apart in a first step. The lid may then be lifted hydraulically or pneumatically and moved to the side.
The clamps can only be removed if pressure measuring devices at the top and the bottom of the vessel indicate that the vessel is no longer under pressure. Then a valve (‘bleed valve’) needs to be opened so as to exhaust gas, which may still be contained in the vessel, to the atmosphere. This process needs to be monitored by the operator. The escape of gas, which is audible or visible for instance by means of an inflated bellows, guarantees that the line is not clogged. This valve needs to be open for some time before the clamp may be unlocked.
An insert basket consists of a tube having flanges at the top and at the bottom, with screens and support plates being clamped therein. In order for the CO2 solvent to flow through the basket, a seal is disposed on the outside for sealing the basket towards the extractor so as to prevent the CO2 solvent from passing into the region between basket and extractor and thus past the substance to be extracted. The insert basket is not subject to mandatory testing.
When using baskets, safety problems may arise if the screen plates become clogged. In such a case, the internal volume of the basket is cut off from the surrounding volume in the vessel; as described above, however, the absence of an internal pressure is only determined on the basis of the pressure measurement inside the volume of the vessel. When the screen plates are clogged it may therefore occur that attempts are being made to open the quick-release fastener although the pressure relief has only occurred in the vessel.
If it has been possible for the clamps and the lid to be opened successfully, an unknown internal pressure remains in the insert basket. The insert basket then acts as a pressurized pressure vessel. The pressure can be relieved by releasing the flanges of the insert basket. If screw flanges are used, a targeted relief of pressure can be performed. In any case, there is always a certain surprise effect as the remaining pressure in the basket might have gone unnoticed.
If the pressure is however relieved via a defect in the basket, this may be extremely harmful to be operator. If for instance the lower bottom flange breaks away when the basket is still in the vessel, this may have a rocket-like effect on the basket, resulting in a behavior of the basket which is similar to that of a pressurized gas cylinder when a valve breaks away.
Similar effects must be expected if the basket bursts while being removed from the vessel.
These potential risks can be avoided if provisions have been made to prevent the screens from becoming clogged so that the internal pressure in the insert basket is always equal to that in the vessel. This can be achieved by not filling pasty substances into the insert baskets. Many years of practical application have shown that screen plates do not become clogged by bulky or powdery materials, especially if the mesh width of the screen plates is large enough.
It may however be necessary to use extremely fine filters instead of the screen plates so as to prevent an entrainment of sludge. Likewise, it may be desired to further investigate the extraction of pasty materials for new product developments.
In this case, measures need to be taken in order to avoid the above-discussed risks whenever there is a differential pressure between insert basket and vessel. For instance, the insert baskets may be provided with predetermined rupture points such as safety valves and rupture discs. It must however be ensured that devices of this type are practically applicable as well, i.e. an early rupture thereof must be avoided. In other words, they must not challenge the success of the extraction process. As a result, the desired pressures would be so high that hazards caused by an internal pressure cannot be excluded. Another huge uncertainty factor is that the mentioned devices may become clogged, thus giving a false effect of safety.